We determined in preliminary studies that the responses to cutaneous heat stimuli of nociceptive primary afferents in the monkey were substantially affected by the choice of anesthetic. These results challenge the commonly held assumption that general anesthetics do not alter the response of primary afferents. The purpose of this proposed research is to characterize further the effect of anesthetics on the response properties of primary somatosensory afferents. A major objective will be to determine which anesthetic has the least effect on the responses of nociceptive afferents and thus can be used to model the awake animal in neurophysiological studies. Using single fiber recording techniques, we will examine the responses of nociceptive afferents not only to heat but also to mechanical stimuli. We will also determine if other types of primary afferents including slowly and quickly adapting low-threshold mechanoreceptive afferents, warm fibers, and cold fibers are affected by anesthetics. Finally, we will determine if ancillary factors such as blood pressure, core and skin temperature, sympathetic tone, and acid-base balance contribute to the anesthetic effect. This research is fundamental to advancing knowledge of the somatosensory system, and in particular pain sensation. This research may also provide further clues as to the mechanism by which anesthetics affect the nervous system.